This invention relates to an ultrasonic testing method for detection of flaws in component parts having a curved surface portion with a specified radius of curvature, and more particularly to an ultrasonic testing method wherein a center axis of curvature of the curved surface portion of the component part and the center axis of a probe are set in an eccentric relationship.
As a testing method for detection of flaws in steel materials, steel plates, forgings, etc., there has conventionally been used an immersion type ultrasonic testing method.
The conventional immersion type ultrasonic testing is generally used where the material to be tested is comparatively large and the flaws to be detected are as large or larger than several millimeters. Therefore, the testing system used for the conventional ultrasonic testing is not a special one. Also, the probe used for transmission and reception of an ultrasonic wave in the testing is an ordinary one, namely, an immersion type probe which has a simple planar oscillator having a large diameter.
Additionally, some products to be used under such severe conditions are such that even minute flaws, if any, would cause problems in practical use of the products. Therefore, a system for ultrasonic detection of even minute flaws ranging down to several hundreds of micrometers in size has been adopted, in order to enhance reliability of flaw detection.
In the ultrasonic testing method for detecting minute flaws, a focus-type probe in which a concave resin lens is attached to the above-mentioned planar oscillator or in which the oscillator itself is shaped to be concave has been used.
In addition, studies have been made in recent years on the use of ceramics for bearing members and the like, which are required to have particularly high reliability.
Because ceramics are brittle materials, a testing method with high resolution has been desired for detection of flaws in the ceramic products. In the relatively new art of ultrasonic testing for detection of flaws in ceramics and the like, therefore, attempts have been made to enhance the sensitivity and accuracy of flaw detection by elevating the test frequency from the previous values of about 0.5-10 MHz to higher values of 15-100 MHz or by using a computer to perform image processing, and other methods.
In application of the above-mentioned prior art, however, the materials to be tested has been limited to comparatively large products of simple shapes such as a flat plate, circular cylinder, prism, circular tube, etc. Also, the flaws which are typically detected have been limited to those flaws being not smaller than 0.5 mm, at best, and located at a depth of several millimeters or more from the surface of the material under test. Accordingly, an attempt to detect the flaws in the surface and the sub-surface of a product having a radius of curvature of 10 mm or less, such as a bearing ball, by the prior art has failed because of scattering and reflection of the transmitted ultrasonic wave at the surface of the product and because of complicated refraction of the propagated ultrasonic wave.
An ultrasonic testing method for detection of minute flaws in the surface, and within a depth of 2 mm from the surface, of bearing rolling elements in the form of balls, cylinders or the like has been proposed in Japanese Laid-Open Patent Application (KOKAI) No. 63-243751 (1988) corresponding to U.S. Ser. No. 07/172,244 now abandoned. In this method, a focus-type probe and the rolling element as the material to be tested are disposed with a predetermined amount of eccentricity therebetween, and ultrasonic flaw detection is carried out to detect minute flaws in the surface and the sub-surface of the material. The method, however, is not applicable to materials to be tested which have a radius of curvature of not more than 10 mm or which have a special curved surface.